This invention relates generally to ceramic tile manufacture.
More particularly, the invention relates to the production of ceramic articles by a production system usually known as a dry system, ie by pressing at least one ceramic material in powder form having a low moisture content, of the order of 3-7%.
More specifically, the invention concerns a manufacturing method, and its means of implementation, by which within one and the same production stage prior to firing, tiles of even considerably different formats can be obtained.
To form said tiles by the dry method it is known to use specific ceramic moulds having one or more forming cavities, depending on the required tile dimensions.
In the case of small formats the forming cavities are arranged in two or more adjacent rows extending transversely to the tile discharge direction.
To vary the tile format, the mould has always to be changed.
For this purpose the mould is removably positioned on a respective operating press having a power adequate for the production underway.
With the press there is also associated a loading unit (for at least one material in powder form) which if the mould is changed also has to be changed, or at least adjusted.
The widespread use of such a manufacturing system has highlighted various drawbacks, of which the following can be mentioned.
The presence, within one and the same works, of presses of different power and forming moulds for producing tiles of different formats results in a considerable initial cost plus ongoing costs for their maintenance and fitting.
Moreover, the need to change the moulds each time the format is to be changed, with consequent changing, or at least adjustment, of the discharge unit associated with the respective press, results in a loss of production, this being particularly negative economically considering the high production rates which characterise modern production cycles.
In addition, when using moulds for tiles of relatively small format, in the form of two or more adjacent rows of forming cavities, it is normally necessary to reduce the press operating speed, and it is quite common to encounter unwanted interference between the tiles directed to discharge.
Finally, a production system using the dry method is known in which slabs of relatively large format are formed by pressing at least one powdered ceramic material, to be then fed to the next stages of the production cycle, fired and finally transformed into tiles having the required dimensions and shape.
This system has however proved unsatisfactory because the transformation requires a relatively very lengthy operating time, it results in particularly rapid wear of the means used for the transformation, and consumes relatively very high absorbed power.
The main object of this invention is to provide a method, as defined in claims 1 to 18, and the relative plant for its implementation, as defined in claims 19 to 35, by which the aforesaid problems are eliminated.
According to a first aspect of the invention, the method comprises the following operational stages: forming within a mould, by dry-compressing at least one ceramic material in powder form, a panel of relatively large dimension, then separating from said panel, after its formation but before its firing, at least one tile having the desired format.
Preferably the panel is dimensioned to comprise a multiple of a given tile format plus the scrap deriving from said separation.
In a variant, the panel dimensions are such as to provide at least two tiles of different format plus the scrap.
According to a preferred embodiment, the starting panel is of right form, for example square or rectangular, in a variant the panel having the shape of a regular polygon, such as a lozenge, rhombus or hexagon.
Said separation comprises at least one cutting operation, said at least one cutting operation being performed by a mechanical means.
Alternatively, said at least one cutting operation is performed by a fluid means, such as air or water under high pressure, with a fine strongly abrasive material preferably mixed with it.
Again, said at least one cutting operation can comprise a first stage of surface incision, and a second stage of actual cutting.
Preferably said cutting, or said surface incision stage, includes the simultaneous formation, along the entire length of the sharp edges of said incision, of a profiled edge having the characteristics and function of the usual edging on the exposed face of a tile.
The same cut can also provide for forming, along the total length of the lower ends of the sides of the cut, small projections functioning as the usual spacers provided along the sides of a tile.
A network of small grooves is preferably formed on the rear face of the panel during its pressing, in correspondence with the lines along which said cuts are made.
The cuts themselves are made in directions parallel to at least one main directrix of the starting panel, for example one side thereof.
Preferably, the cuts are orientated along two mutually perpendicular horizontal directions, typically the direction parallel to the panel discharge direction and the direction perpendicular thereto, although other different directions, rectilinear or non-rectilinear, can be chosen, as will be apparent hereinafter.
Finally, in a first embodiment, said separation takes place immediately after forming the panel in the press, whereas in a further embodiment, separation takes place after at least partial drying of the panel.
According to a second aspect of the invention, the means for implementing the aforesaid method comprise a press of large power, such as a hydraulic or mechanical press, with which there is associated a single-exit ceramic mould arranged to form a starting panel of large dimensions, and at least one sectioning unit provided downstream of said press to obtain from said starting panel at least one tile of the desired format.
Said at least one sectioning unit is associated with a conveyor for removing the panels discharged from the mould, and comprises at least one cutting tool arranged to divide the panel into at least two equal or non-equal parts.
Said at least one cutting tool can be located in a fixed position above the moving conveyor, in such a manner as to cut the panel while it advances on this latter.
Alternatively, the said tool can be made to traverse across the at that moment stationary panel in any direction, as explained hereinafter.
In a first embodiment, said at least one cutting tool comprises a single element of horizontal axis, such as a diamond-set wheel or a disc blade with inserted teeth.
Alternatively, said at least one cutting tool can consist of a vertical nozzle delivering a fluid under high pressure, such as air or water.
Preferably a highly abrasive fine powder, such as powdered glass, sand, powdered metal, ground tile powder, industrial diamond particles or the like, is mixed with the fluid.
Alternatively, said at least one cutting tool comprises two members, such as two diamond-set wheels or two disc blades of horizontal axis, positioned in succession along the cutting direction, one of which incises the upper surface of the panel to a greater or lesser depth, and the other completes its cutting.
The incision tool is preferably thicker than the cutting tool, and is preferably shaped to create an edging along the upper sharp edges of the cut.
Typically for obtaining tiles of relatively small format, the means of the invention comprise two sectioning units, each provided with at least one cutting tool of the stated type, the units being positioned in succession along the panel advancement direction.
Said two units are arranged to operate along two different horizontal directions, for example mutually perpendicular, typically parallel to and perpendicular to said advancement direction respectively.
Advantageously, if the cutting tool is a nozzle or, alternatively, at least one cylindrical or conical motorized cutter or wheel of vertical axis as clarified hereinafter, said tool can be under the control of a robotized system which can operate with the panel either temporarily at rest or in movement.
The means of the invention can also be provided with a device for at least partially drying the panel before subjecting it to said sectioning.
Finally, it should be noted that the invention also comprises the tiles obtained by said method and said means or plant.
From the aforegoing, it is apparent that all the objects are attained, in that:
tiles with even considerably different formats can be obtained from one and the same mould and a single press,
it is not necessary to change the mould when the tile format changes, or to change or adjust the discharge unit associated with the press, and
when tiles of relatively small format are produced, there is no undesirable interference between them.
Moreover as the panel is sectioned after its pressing, or at most after at least its partial drying, ie before firing, the cutting means used have a relatively long useful life, the power absorbed by them is fairly low, and their operating time is acceptable, as has been found by specific tests carried out with plants in accordance with the teachings of the invention.